diff -r b92f2abe0bda -r dcf38ac6a38c net/server_event.lua
--- a/net/server_event.lua	Mon Aug 16 11:37:51 2021 +0200
+++ b/net/server_event.lua	Mon Aug 16 12:34:52 2021 +0200
@@ -449,6 +449,7 @@
 	self.onstatus = listener.onstatus;
 	self.ondetach = listener.ondetach;
 	self.onattach = listener.onattach;
+	self.onpredrain = listener.onpredrain;
 	self.ondrain = listener.ondrain;
 	self:onattach(data);
 end
@@ -464,6 +465,8 @@
 end
 function interface_mt:onreadtimeout()
 end
+function interface_mt:onpredrain()
+end
 function interface_mt:ondrain()
 end
 function interface_mt:ondetach()
@@ -490,6 +493,7 @@
 		onincoming = listener.onincoming;  -- will be called when client sends data
 		ontimeout = listener.ontimeout; -- called when fatal socket timeout occurs
 		onreadtimeout = listener.onreadtimeout; -- called when socket inactivity timeout occurs
+		onpredrain = listener.onpredrain; -- called before writes
 		ondrain = listener.ondrain; -- called when writebuffer is empty
 		ondetach = listener.ondetach; -- called when disassociating this listener from this connection
 		onstatus = listener.onstatus; -- called for status changes (e.g. of SSL/TLS)
@@ -540,6 +544,7 @@
 					interface.eventwritetimeout = false
 				end
 			end
+			interface:onpredrain();
 			interface.writebuffer = { t_concat(interface.writebuffer) }
 			local succ, err, byte = interface.conn:send( interface.writebuffer[1], 1, interface.writebufferlen )
 			--vdebug( "write data:", interface.writebuffer, "error:", err, "part:", byte )